1. Field of the Invention
The present invention relates to a branching filter for using a single antenna with a plurality of spectra, and a multiplex transceiver which utilizes the branching filter.
2. Description of the Related Art
Conventionally, a microwave branching filter device has been developed, as shown in JP-B-3178434. This microwave branching filter device comprises an antenna for transmitting and receiving signals, reflecting means for reflecting an applied signal, a plurality of transmission/reception duplexers, and the same number of circulators as the plurality of transmission/reception duplexers. Then, each of the circulators is disposed between the antenna and reflecting means. In particular, two of the three terminals of the circulator are connected in series such that they are connected to different terminals from each other, to allow an incoming signal from the antenna to flow to the reflecting means. Each of the plurality of transmission/reception duplexers has its duplexer terminal connected to the remaining terminal other than the aforementioned particular two terminals of each circulator, a transmission terminal connected to a transmission filter, and a reception terminal connected to a reception filter.
Apparent from the configuration described above, the transmission filter and reception filter, which are band pass filters, comprise one port on the antenna side with the terminal of the transmission filter connected to the terminal of the reception filter, and two ports on the radio device side with the terminal of the transmission filter not connected to the terminal of the reception filter.
Recently, radio devices used in microwave networks and the like rapidly decrease in size. This type of radio device is designed in a transmission/reception integrated configuration which has a transmitter and a receiver incorporated in a single housing to transmit an outgoing signal from the transmitter to an antenna and receive an incoming signal from the antenna to the receiver through a single port. Then, for making communications through the transmission/reception integrated radio device, single transmission/reception integrated radio device 1 is combined with single antenna 2, and they are connected through single port P, as illustrated in FIG. 1.
In communications using a microwave network, signals from a plurality of radio devices are often multiplexed for transmission and reception. While transmission/reception integrated radio device 1 is configured to transmit an outgoing signal from the transmitter to the antenna and receive an incoming signal from the antenna to the receiver through one port, this radio device 1 may be connected to the microwave branching filter device shown in JP-B-3178434 for use therewith.
The microwave branching filter device disclosed in JP-B-3178434 employs band pass filters for the transmission filter and reception filter, wherein the terminal of the transmission filter is connected to the terminal of the reception filter to provide one port on the antenna side, while the terminal of the transmission filter is not connected to the terminal of the reception filter to provide two ports.
Therefore, for connecting a transmission/reception integrated radio device to the microwave branching filter device disclosed in JP-B-3178434, an additional branching filter must be installed for separating an outgoing wave from a incoming wave between the two ports on the radio device side of the microwave branching filter device, i.e., the ports of the transmission filter and reception filter, and the one port of the transmission/reception integrated radio device.